Attenuated type II poliovirus P712 Ch2ab cannot grow at 40 degrees C whereas the virulent Lansing strain of type II poliovirus grows equally at 35 degrees C and 40 degrees C. Studies in progress show that at 40 degrees C peptides from 12,000 to 14,000 MW are produced by P712 Ch2ab. At 35 degrees C peptides with molecular weights of 120,000 to 12,000 are synthesized by attenuated virus. Several studies indicate that at 40 degrees C there is premature termination of translation with attenuated but not virulent virus. First, inhibitors of cleavage such as tosyl-1-lysinechloromethyl ketone and amino acid analogs induce large peptides of 180,000 MW at 35 degrees C but not at 40 degrees C. Amino acid ratio studies show that peptides formed at 40 degrees C are different than those produced at 35 degrees C. Finally, Edman degradation studies show initiation to be normal at 40 degrees C. All in all the data strongly suggest premature termination of protein synthesis at 40 degrees C with attenuated but not virulent virus.